1. Field of the Invention
The present invention relates to a stepping motor having a rotor disposed to surround a stator and a rotor for the stepping motor.
2. Related Art
A stepping motor has a simple structure and is easy to control and is therefore used in various fields. Especially, a PM (Permanent Magnet) stepping motor using a permanent magnet can be manufactured at low cost and is therefore used frequently in various fields.
Reliability is important in this type of stepping motor and it is especially required of the stepping motor to start up normally under all conditions. To ensure that the stepping motor starts up normally, it is necessary to control the stepping motor to stop at an exact predetermined position. For high-precision stop position control, adjusting detent torque to be small is common practice. This is because the detent torque is a load in rotation.
However, if the load is heavy, e.g., if a weight is used as a load as in a stepping motor for generating vibrations, it is difficult to precisely stop the motor at a desired stop position when the motor is not energized. Therefore, there have been proposed techniques for reliably stopping the load by increasing detent torque when the motor is not energized (see Japanese Patent Application Laid-open No. 43059/1985, No. 78513/1994 and No. 308214/1997).
Among the PM stepping motors, an outer-rotor type single-phase stepping motor has a rotor disposed to surround an annular stator. The stator has stator yokes formed with a plurality of pole teeth and the rotor has a magnet in which magnetic poles having different polarities are disposed alternately along a circumference thereof.
It is known that detent torque (cogging torque) changes by adjusting a magnetization waveform of a magnet, i.e. a surface magnetic flux density distribution waveform of the magnet. As the magnetization waveform is close to a sinusoidal wave, harmonic components decrease, thereby reducing a detent torque. Therefore, it is a common practice to adjust the magnetization waveform to be close to the sinusoidal wave.
For example, in Japanese Patent Application Laid-open No. 2001-57752, a back yoke is disposed to surround a magnet and is provided with recessed portions along boundary positions between magnetic poles of the magnet. The boundary positions between the magnetic poles of the magnet face the recessed portions. Therefore, flux content changes slowly at the boundary positions of the magnetic poles and the detent torque can be reduced.
In fact, in a structure for reducing the detent torque, it is possible to precisely perform stop position control. However, there is no guarantee that the stop position control can be precisely carried out when the motor is not energized. A shaft of a rotor may rotate even when the motor is not energized. In such a case, it is preferable to carry out the stop position control for stopping the rotor in a predetermined position. The reason is that if the rotor stops in an arbitrary rotating position when the motor is not energized, start-up performance is deteriorated when energization is started up after that. However, the above-described Patent Documents 1 to 4 do not particularly consider the stop position control at non-energization time of the motor.